A radio telephone network is made up of a number of terrestrial radio base stations, which provide radio coverage for zones defining the respective cells. To be able to establish communication between the mobile radio handsets in a cell and the station, the receiver circuits in these must receive signals above their sensitivity threshold.
The electronic noise from receiver circuit components adds to the radio propagation noise and may make the link unusable, for example, on a digital link, by causing incorrect detection of certain of the bits transmitted. Whatever the type of modulation used, the risk of errors in detecting bits rapidly increases when the level of useful signal received, and therefore the signal to noise ratio, descends to a threshold value only offering a minimal safety margin for noise interference.
Encrypting transmissions of data bit packets using redundancy, with a self-correcting code, enables the receiver to detect errors and to correct them if they are not too numerous. This enables an additional limited reduction in the level of reception to be tolerated, but beyond that, the number of excessive errors prevents any correction. Even if the logical link with the station can initially be established and maintained by service signals, to reserve a useful transmission path, the excessive rate of incorrect useful bits makes the latter unusable, for digitised voice and for other data.
In a general manner, the stations and the handsets must be equipped with highly sensitive receiver circuits, i.e. with a relatively low detection threshold, and powerful transmitter circuits.
For reception, the circuit components used are selected because they only generate a low level of intrinsic noise. It is possible therefore to amplify the useful signal received, until the level of intrinsic noise, amplified with the useful signal, starts to become disruptive. The sensitivity threshold for the receiver is reached. Distortions of radio propagation also represent a cause of errors, limiting the possibilities for detecting weak signals.
When transmitting, the power of the stations must remain limited, to prevent disruption between the cells and other radio systems. The transmission power of the handsets must also be limited, to maintain the autonomy of their batteries and to protect the health of users.
The operator can therefore not guarantee that all the handsets in a cell will receive a signal of a level at least equal to the sensitivity threshold to provide a radio link, in particular because of obstacles, such as walls in a building or the bodywork of a car carrying the user. The same applies in the other sense, to the station.
In practice, as the strength of the radio field decreases with distance, according to a recognised attenuation law, the size or “range” of the cells is limited to the distance for which the level of the field has markedly decreased to the sensitivity threshold for the handsets, with nevertheless a pre-determined safety margin to take account of temporary obstacles to radio propagation, causing a loss of signal.
The network operator also takes account of the contours of the cell and the type of environment, buildings, woods and other features, to correct the calculations of the field level performed using the attenuation law. Using these calculations, he determines the masked zones in the cell, i.e. the zones in which the level of reception of signals from the station is lower than the sensitivity threshold for the receivers.
In other words, and if each cell is divided into a grid of a number of pixels each representing an area of several tens or hundreds of square meters, the radio coverage map is made up of a mosaic of “black” or “white” pixels, depending on whether the field for the station is respectively higher or lower than the correct functioning threshold for handsets. The problem of the link to the station is less critical because this is balanced by parameterising the transmission conditions and the design of the network with the outgoing link.
The applicant has tackled the problem of the validity of such a binary approach for representing, as described above, or in another fashion, the coverage map for a radio telephone service.
He has observed that the above method for determining the existence of radio coverage does not take into account the dynamic aspects of radio propagation, i.e. changes in the latter over the course of time, and therefore alternating availability of a radio link and non-availability in the zone represented by each pixel, depending on fluctuations in the level received around the sensitivity threshold of the receivers.